THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 215 
power in the model, the close agreement between the performance of the ship on trial 
and the estimates from the model tests must be taken as a remarkable confirmation of 
the conclusion that the law of comparison applies to propellers as actually fitted on ships. 
Now it must be admitted that the foregoing results have been obtained by the 
application of theory and, as has been shown before, the extensions from the model to 
the ship could not have been easily made by empirical methods and certainly not by the 
particular method used by Admiral Dyson. 
I do not mean to say by this that Admiral Dyson’s method has not given valuable 
results in designing propellers for ships, but I think that the enormous amount of val- 
uable data which he has accumulated and which is presented in his paper could be made 
more readily available to the ordinary man if it were coupled with a theory such as that 
of mechanical similitude. 
It seems to me that the next step in the development of our knowledge of the action 
of propellers, and the putting of this knowledge into suitable form, would be to find 
the factors connecting the formulae derived by theoretical means with the empirical 
facts of experience. When this is done I believe that the design of propellers will be 
very much simplified, and the student will have fewer obstacles to contend with in acquir- 
ing a sufficient knowledge of the subject. What is really needed is not a system of pro- 
peller design based on theory alone, nor one which is based on empirical methods alone, 
but one which will combine the good features of each method as well as eliminate the 
weakness of each. 
I trust the author of this paper, who has deservedly obtained a wide reputation 
and is rightfully considered to be one of our leading authorities on the subject, may 
find time to connect his empirical method with sufficient theory to accomplish a happy 
blending of theory and practice and thus make his results more readily accessible to 
naval architects and marine engineers. 
Mr. E. A. STEVENS, JR., Member:—For the past eight years I have followed with 
no small amount of interest various articles published in the Journal of the American 
Society of Naval Engineers, written by Admiral Dyson, and have noticed the advances 
made from time to time, especially regarding the determination of the slip block coefficient 
and the Power Augment Factor K. I must confess that I was somewhat surprised to 
see the difference in the way of determining these factors (especially the S. B. C.) in this 
article and the method as suggested in the Marine Engineers’ Handbook, in which no 
attempt is made to correct for the ratio of draught to beam (H+B). 
I have not had time to study this paper carefully but, on looking over it, the follow- 
ing points appear to need a little explanation :— 
1. Is it to be understood that, in determining the slip B. C. of single-screw ships 
of all three types, no correction for variation of midship-section coefficient is to be made? 
And does this also apply to twin-screw ships of types 1 and 2? 
2. Is there no correction to be made for variation of midship-section coefficient 
in arriving at the K block coefficient? 
3. Would it not be as accurate and less complicated to use the prismatic in place of 
the block coefficient? 
4. To what type would high-speed yachts, torpedo boats, and the earlier destroyers 
belong? The fining of the after body of these vessels is produced neither by a rapid 
dead rise of the after sections nor by a rapid decrease of beam, but by the raising of the 
